High pressure processing (HPP), also known as high hydrostatic pressure processing, pascalization, and bridgmanization, is a technique whereby articles such as food items are subjected to pressure of an intensity and for a duration of time sufficient to reduce the biological activities of cells and their components, thereby decreasing the likelihood that cells subjected to such processing will continue to metabolize or reproduce. As such, HPP techniques can be used to mitigate or eliminate the risk that bacteria, mold, yeast, and parasites will survive or multiply in or on HPP-treated articles.
Foodstuffs intended for human or other animal consumption are susceptible to microbial contamination, especially during handling and processing, and such contamination can continue and increase during storage. Furthermore, natural components (e.g., endogenous enzymes) in foodstuffs such as vegetables, fruits, and meats can exert degradative effects upon the foodstuff during storage, even in the absence of microbial contamination. A continuing need exists for methods of reducing spoilage and degradation of foodstuffs during storage. HPP methods have been widely investigated and reported. Their efficacy for reducing the risk of contamination of foodstuffs and thereby extending the period for which foodstuffs can practically be stored has been demonstrated by others.
In HPP techniques, food or other articles are often sealed in a container prior to subjecting the sealed container to high pressure, such as 200-1000 megaPascals (MPa; more typically 200-600 MPa). Because gases are highly compressible at such pressures, some or substantially all gases can be removed from the container prior to sealing it, such as by evacuating gases from the container or by filling the container with liquid prior to sealing it. Liquids and solids, being relative incompressible at these pressures, tend to transmit pressure throughout their volume, provided there are no rigid articles present (e.g., thick, hollow bones or shells capable of preventing transmission of isotropically applied pressure to their interior). Pressure can be applied to the exterior of the article or the exterior of the container containing the article and transmitted throughout the article. In practice, pressurization is often achieved in a sturdy device designed for accommodating one or more articles during such pressurization.
Pressurization devices often have controls for selecting the pressure and the duration of pressurization and indicators for indicating the pressure achieved and/or the duration of the pressurization process.
HPP processes do not require heating of the treated article, although heat can be applied, for example to inactivate spores or other heat-labile components that can be expected to be unaffected by HPP processing.
Following HPP treatment, non-compressible articles such as liquid foods (e.g., soups and juices) often appear similar to or indistinguishable from non-treated articles of the same type. Although failure of HPP treatment to alter the outward appearance of an article may be beneficial in some respects (e.g., in that treated foods may look identical to fresh, untreated foods), the lack of apparent changes can lead to confusion among HPP-treated and -non-treated articles. In an environment, such as a food processing facility, in which non-treated articles can be expected to be in close proximity to non-treated articles, the appearance of the articles alone can be an insufficiently reliable indicator of HPP treatment status to prevent an operator from mistaking a non-treated article for an HPP-treated article. Likewise, because pressurization equipment can sometimes fail to achieve the degree or duration of pressurization desired by its operator, and such failures can be difficult for an operator to perceive. For articles that fail to undergo an alteration in their appearance during HPP processing, such failures may go undetected and insufficiently processed articles can be mistaken for appropriately processed ones.
It would be beneficial if an indicator could be associated with an article intended to be subjected to HPP, the indicator undergoing a readily-observable change of appearance upon HPP and not undergoing the change absent HPP.
Others have recognized the need for such an indicator.
By way of example, U.S. Pat. No. 6,799,467 to Minerich et al. discloses a tablet formed from powdered metal that is capable of undergoing a change in density upon HPP. A potential drawback of such a tablet, however, is that determining a change in tablet density can be difficult and time-consuming to detect, and outward changes in the tablet may not be readily discernable.
Further by way of example, European Patent Application publication no. 2116481 discloses an irreversible pressure-sensitive marker in which pigment-containing microcapsules are disposed between a rigid base member and a transparent film. When subjected to HPP, the rigid base member retains its shape and anisotropic force is applied to the microcapsules by the transparent film, resulting in rupture of the microcapsules, escape of the hitherto-contained pigment, and visibility of the pigment through the film. In view of the high pressures used during HPP and the corresponding expense of providing a rigid base member capable of retaining its shape under such pressures so as to exert anisotropic force upon the microcapsules, such a system can be economically impractical for common use.
Disclosed herein is a sensor and method of using it that can be associated with an article to be subjected to HPP. The sensor undergoes a readily-observable change of appearance upon HPP and does not require a rigid base member.